One might think that a pair of SLI cards with 640 MB of memory between them would have all the memory they needed, and that a pair of the same GPUs with 1280 MB of memory between them would be overkill- well surprisingly, the tests showed a consistent performance advantage for the two 640MB cards over the two 320s- The advatage of double memory probably wouldn't be universal in previous generation video cards, though, as the slower processor speed of these cards is not able to take advantage of as much extra memory.

"With in excess of 1.2GB of memory on offer when the 640MB cards are enabled in SLI mode, you have to wonder are we simply at a point of overkill. Are the 640MB cards for non-SLI and the 320MB cards for SLI? These are some of the questions we hope to answer today."

considering that the 8800GTX come with 768MB of RAM, and new cards coming out will have 1GB of RAM on them, i think that having 512MB is the new minimum. although really on these middle to lower end cards, i always question when they have a lot of RAM, often slower RAM, and don't have the GPU to really make use of it all.